A conventional semiconductor device is known, which has a sapphire substrate, a Cl-containing AlN buffer layer formed on the sapphire substrate, a Cl-free AlN buffer layer formed on the Cl-containing AlN buffer layer, and a Cl-free device constituent layer formed on the Cl-free AlN buffer layer (see, e.g., PTL 1).
In the semiconductor device described in PTL 1, the Cl-containing AlN buffer layer is formed by the Hydride Vapor Phase Epitaxy (HVPE) method using chloride as a raw material and thus contains Cl, while the Cl-free AlN buffer layer is formed by the Metalorganic Chemical Vapor Deposition (MOCVD) method not using chloride as a raw material and thus does not contain Cl.
According to PTL 1, the Cl-free AlN buffer layer formed by the MOCVD method prevents diffusion of Cl from the Cl-containing AlN buffer layer formed by the HVPE method and thus prevents an increase in contact resistance of an ohmic electrode.
The Cl-containing AlN buffer layer described above is formed under specific growth conditions and is thus considered to have uniform Cl concentration distribution.